Conventionally, a balun, hybrid, or power combiner is provided at the output side of a balanced output circuit, particularly, a push-pull amplifier (having a pair of amplifiers that operate at a phase difference of 180°). The balun or the like converts a balanced signal into a single-ended signal.
In generally, baluns are used in the microwave band and lower (the HF band, VHF band, UHF band, and lower). On the other hand, hybrids or power combiners are used in the microwave band and higher (the UHF band and higher). Wideband ferrite cores are often used for the baluns, in which case, the available frequency upper limit is the UHF band. Typically, hybrids or power combiners are configured with distributed-constant circuits. Thus, the sizes do not cause a significant problem in practice, for the UHF band or higher.
Meanwhile, in a communication apparatus, particularly, in a transmission circuit section for QPSK or the like involving amplitude modulation components or in a transmission circuit section that requires high reliability, a transmission signal converted into a single-ended signal is sent through an isolator and then through an antenna switching device (or antenna duplexer) and the resulting signal is sent to an antenna. Unless the signal is sent through the isolator, reflections from the antenna and the antenna switching device return to a balanced output circuit (especially, amplifiers), thereby varying load impedance viewed from the balanced output circuit. When the load impedance varies, some problems arise. For example, the waveform distortion of the transmission signal becomes severe and the operation of the amplifiers becomes unstable for oscillation.
However, when a balun (or a hybrid or a power combiner) and an isolator are combined as in the conventional manner, the size and cost of the transmission circuit section increase, thereby making it impossible to meet recent demands for miniaturization and cost reduction of mobile communication apparatuses. Also, since a transmission signal passes through both the balun and the isolator, the insertion loss is increased. Further, since the transmission circuit section handles a large amount of power, when the number of connection portions increases as a result of an increase in the number of components, there are problems in that unwanted radiation is easily produced and the possibility of mutual interference in the communication apparatus increases. In addition, since the operating bandwidths of the balun and the isolator reduce the operating bandwidth of the transmission circuit section, there is a problem in that the available frequency band is reduced.
Accordingly, as described in Japanese Unexamined Patent Application Publication No. 2002-299915, the present inventor proposed a nonreciprocal circuit device that can be connected to a balanced output circuit without a balun, hybrid, or the like interposed therebetween and also proposed a communication apparatus. In the nonreciprocal circuit device, two opposite ends of a center electrode for one port are configured as hot ends, and balanced input and unbalanced output are employed. The nonreciprocal circuit device can be used to combine outputs from a push-pull amplifier. However, with the circuit device, in the case of a low operating point (an operating point at which the amount of bias current, i.e., idling current, is small), reverse-phase excitation current does not effectively flow through a reverse-phase-side input terminal. Thus, a problem has been found in that the ferrite excitation efficiency is insufficient.